A grass mowing machine may include a motor or engine that transmits power to left and right traction drive wheels through a dual hydrostatic transmission. The operator may use a pair of controls connected to the dual hydrostatic transmission to control machine speed and direction. Examples of grass mowing machines with dual hydrostatic transmissions to control speed and direction are zero turning radius mowers, walk-behind mowers, stand-behind mowers and other riding mowers.
A dual hydrostatic transmission may include two hydrostatic pumps in one housing, each pump having an input control or trunion shaft. Each input control or trunion shaft may be operatively connected to a swash plate in one of the pumps. Each pump may be connected to a hydrostatic motor that turns a drive wheel. Each input control or trunion shaft may be pivoted in a first rotary direction to move the swash plate in the pump for propelling a traction drive wheel on the vehicle forward, and may be pivoted in an opposite rotary direction to move the swash plate and propel the traction drive wheel in the reverse direction. Each input control or trunion shaft and swash plate may have a forward drive pivot direction, an opposite reverse drive pivot direction, and a neutral position between the forward drive and reverse drive pivot directions.
A dual hydrostatic transmission may include operator controls and linkages connected to each input control or trunion shaft to allow an operator to independently change the direction and speed of each drive wheel. For example, each operator control may be a pivotable stick or foot pedal, and the operator may use the pair of sticks or foot pedals to control speed and direction of the machine. Additionally, each linkage may include a return to neutral mechanism that urges the input control or trunion shaft and swash plate toward the neutral position when the operator releases the stick or foot pedal. Optionally, each operator control also may have a damper to help control motion.
Return to neutral mechanisms may be connected to electrical circuits that prevent the engine or other power source from being started if the transmission is in a driving, non-neutral mode, and thereby generally prevent the vehicle from lunging forward when started. Examples of return to neutral mechanisms are shown in U.S. Pat. Nos. 6,935,106, 5,586,955 and 4,531,365.
Return to neutral mechanisms for dual hydrostatic transmissions require numerous mechanical linkages and other components. The device must return the pump to mechanical neutral from both the forward and reverse motion positions. A return to neutral mechanism for a dual hydrostatic pump is needed having a low number of components.
Mechanical linkages and other components for a dual hydrostatic pump's return to neutral mechanism are costly to assemble, service and adjust in the field. A return to neutral mechanism for a dual hydrostatic pump is needed that is economical to assemble, service and adjust.